It is known that a composite oxide having mesopores is used as one example of the porous substance in an exhaust gas purifying catalyst. In JP-A-2001-170487, for example, there is described a porous substance, which is used as the exhaust gas purifying substance having such a structure but not a substantially fibrous structure that the pores have a center diameter of 2 nm to 100 nm, and are formed at least partially into a three-dimensional mesh shape and communicate at random with each other. There is also described a method for producing the porous substance, in which a precipitate of a hydroxide of a metal such as aluminium or magnesium is formed and is baked after rinsed and dried. In JP-A-2002-220228, moreover, there is further a method for producing oxide powder of a porous substance, in which a precipitate of precursors of a solid solution of CeO2-ZrO2 is separated out and baked.
In the prior art, on the other hand, a microemulsion method is known as a method for producing a composite oxide to be used as the exhaust gas catalyst. In this method, a reaction such as a hydrolysis is caused inside or at the boundary of micelles or inverse micelles either to produce primary particles or precursors of a porous substance and secondary particles or the agglomerate of those primary particles or to form a precipitate, and the agglomerate or precipitate of the secondary particles is baked after rinsed and dried.
The methods, as described in JP-A-2001-170487 and JP-A-2002-220228, precipitate the metal for a carrier of the catalyst, and are enabled to control the pore diameter and the pore distribution to some extent by adjusting the concentration of an aqueous solution. However, the methods cannot make the separation in a state carrying precious metal particles having a catalytic activation. Therefore, the catalytic particles carried may cause a sintering at the prevailing high temperature, and their activity may become lower.
On the contrary, the microemulsion method forms primary particles of the carrier in the state, where the ions of a metal to function as the catalyst are incorporated into the inside or the boundary of the micelles or inverse micelles dispersed in a solvent by a surfactant, or makes the agglomeration of the primary particles and the secondary particles agglomerated from the primary particles. As a result, the microemulsion method can produce a catalyst of a porous structure suppressing the sintering of particles having a catalytic activity.
In the microemulsion method of the prior art, however, the agglomeration of the secondary particles is caused by the slow collisions of the micelles or inverse micelles resulting from their Brownian movements and by the agglomeration of the secondary particles due to the accompanying fusion by the van der Waals force of the secondary particles. In case the pores are placed, after baked, in a high-temperature atmosphere, therefore, their volume is not especially changed, but their diameter is highly changed. In this sense, the heat resistance is not necessarily sufficient. This phenomenon is believed to come from the following phenomenon. In the case of an exposure to a high temperature, the sintering is promoted at the portion of a high surface energy because the force binding the secondary particles is weak. As a result, the particles are bound to each other while crushing the diametrically small pores. After all, the peak of the pore distribution changes to the larger diameter side so that the substantial surface area of the porous substance is reduced.